Dowels for fireable ceramic dies and method for their use



July 8, 1969 R. w. WALTKE 3,453,736

DOWELS FOR- FIREABLE CERAMIC DIES AND METHOD FOR THEIR USE Filed April 7, 1967 m EAT-1E; EB-'IT;EE.T1E.ED. 1%. 5E.

IN VE/VTOR. ROBERT W. WALT/(E United States Patent 3,453,736 DOWELS FOR FIREABLE CERAMIC DIES AND METHOD FOR THEIR USE Robert W. Waltke, Flushing, N.Y., assignor to Ceramco Industrial Corporation, Long Island City, N.Y., a corporation of New York Filed Apr. 7, 1967, Ser. No. 629,258 Int. Cl. A61c 13/00 US. Cl. 32--40 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Until recently, the construction of porcelain jacket crowns for dental prostheses has required the use of a fabricated metal substrate or matrix which is fabricated by casting or swaging to the dimensions of a die, the die having been prepared by casting gypsum type materials, amalgams, plastics and the like into an impression in wax, rubber or other suitable material which has the shape and size of the tooth preparation to be covered by the porcelain restoration. This procedure, while used extensively, involves a number of intermediate and relatively expensive steps between the preparation of the die and the final porcelain jacket or restoration.

Many of the aforesaid intermediate and expensive steps could be eliminated if the porcelain restoration were constructed directly on the stone model or die. However, until recently, materials which were employed for making models or dies could not be used for direct construction of porcelain jacket crowns and the like since the material used for the die or model were unable to withstand the thermal treatment necessary to glaze the porcelain without loss of physical integrity. Furthermore, the coefiicients of thermal expansion of such materials are such that any porcelain summounting the dies would crack on firing and cooling.

In copending application Ser. No. 583,087, filed Sept. 29, 1966, a castable refractory die composition is described upon which porcelain dental prostheses can be directly fabricated and fired. Thus, all of the intermediate and expensive steps between the initial model and the formation of a metal substrate or matrix are eliminated, and the porcelain restoration can be constructed directly on the stone model or die material.

In forming a porcelain jacket crown, inlay or the like,

a thickness of the tooth structure is first removed by a dentist. Thereafter, a wax, rubber or the like impression is made of the tooth so prepared. In this process, the impression of more than the single tooth is formed. That is, a reproduction of a group of teeth is formed in the wax impression; and, thereafter, a model of the entire group is cast. While the model of an entire group of teeth possibly can be used directly as a base for the fabrication of porcelain jackets on individual teeth, problems of bulk usually require that individual tooth dies be extractable from the gross model. In conventional, nonfirable dental stone models this is accomplished by inserting a solid brass or other metal pin or dowel into the ice,

surrounding material before it sets, then pouring further stone to provide a base. The individual tooth die with the dowel attached is then cut from the gross model, whereupon the dentist can use the cut-away die as a basis for the metal matrix on which the porcelain is constructed without interference from adjacent tooth dies.

Where the material used to form the model or die is of the ceramic type described in the aforesaid copending application Ser. No. 583,087, the usual solid rod-type dowels cannot be used since the ceramic die material must be fired to temperatures on the order of about 2000" F. That is, the ceramic material from which the model or die is formed has a much lower coefficient of thermal expansion than a conventional dowel, meaning that the bulk expansion of the dowel upon heating will produce stresses at the ceramic-dowel interface and, consequently, fracture the ceramic die.

SUMMARY OF THE INVENTION In accordance with the present invention, and as one object thereof, the aforesaid problem of differential thermal expansion between a ceramic die and a dowel is eliminated by providing a dowel of tubular construction.

More specifically, an object of the invention is to provide a dowel for use in refractory models or dies employed in the preparation of porcelain dental prosthetic retorations, which dowel has a relatively low bulk expansivity whereby fracture of the die will be avoided.

Still another object of the invention is to provide a method for forming a model or die for dental prosthesis forming a fireable ceramic material containing a tubular dowel.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIGURE 1 is a perspective view of a ceramic die of a group of teeth, certain of said teeth having dowels inserted therein for subsequent removal;

FIG. 2 is a cross-sectional view of a rubber or wax mold impression from which the model of FIG. 1 is formed;

FIG. 3 illustrates the first step in the formation of the model of FIG. 1 by pouring a fireable refractory material into the mold of FIG. 2 and inserting a dowel therein;

FIG. 4 illustrates the final step in the formation of the model of FIG. 1, comprising pouring stone over the previously-formed refratcory material and dowel;

FIG. 5 is an illustration of an individual tooth die which has been extracted from the gross model of FIG. 1; and

FIG. 6A-6E illustrate various embodiments of the tubular dowel of the invention.

With reference now to the drawings, and particularly to FIG. 1, a stone model of a group of teeth is shown. It will be assumed that the two items identified by the numer als 10 and 12 are models of teeth which have been previously ground to remove a portion of the surface thereof preparatory to the formation of a porcelain prosthetic restoration. The models 10 and 12 as well as the remainder of the teeth in the bridge model are formed from a castable, refractory composition, hereinafter described in detail, to form an upper portion 14 of the overall model. This upper portion 14, will be noted, conforms to the teeth of the bridge as well as the gum portion which supports those teeth. Beneath the upper portion 14 is a lower base portion 16 which, as will be seen, can be formed from conventional stone material utilized in the dental profession.

Projecting through the base portion 16, the upper portion 14 and into the two teeth models and 12 are dowels 18 and 20, hereinafter described in greater detail. As was mentioned above, it is often necessary to remove from the model of the complete group the individual tooth dies 10 and 12; and it is for this reason that the dowels 18 and 20 are included. To remove an individual die such as die 10, for example, saw cuts 22 and 24 are formed in the upper portion 14 and base 16 on either side of the die. Thereafter, and as illustrated by the die 12, the dowel is pushed upwardly whereby the area 26 beneath the die 12 breaks away from the remainder of the overall bridge model, thereby producing the individual tooth die of FIG. 5. Thereafter, this die may be utilized as a base upon which a porcelain jacket crown, inlay, restoration or the like may be formed.

With reference, now, to FIG. 2, the initial step in forming the model of FIG. 1 comprises forcing a wad or mass of soft pliable material such as rubber or wax over the teeth which have been previously ground to form the mold 28 of FIG. 2. The thus-formed mold has a plurality of teeth cavities therein, one of said cavities being indicated in FIG. 2 by the reference numeral 30.

After the mold 28 of FIG. 2 is formed, a castable refractory composition is poured into the mold (FIG. 3) upto the level indicated by the reference numeral 32. As will be understood, this forms the upper portion 14 of the complete model shown in FIG. 1. The castable refractory material 34 is, as mentioned above, of the type described in copending application Ser. No. 583,087. Essentially, it comprises a mixture of magnesium oxide, calcium aluminate, ammonium dihydrogen phosphate and an alkali or alkaline earth fluoride.

These materials, comprising the refractory composition, are mixed with a colloidal silica solution to give a thixotropic slurry which is poured directly into the mold cavity 30 and up to the level 32. After the mixture of refractory composition and colloidal silica solution is thus cast into the impression, it is allowed to remain therein for approximately twenty minutes which is suificient to enable the cast material to set to a hard mass. However, before the hard mass is formed, a dowel 36 is inserted therein above the impressions 10 and 12, for example, shown in FIG. 1.

After the refractory material 34 has thus hardened with the dowel 36 inserted therein, it may be removed from the mold or impression and fired for maturation of the materials system. Thereafter, it may be reinserted into its mold cavity 30 in the impression 28 and a quick-setting, non-refractory stone material 38 (FIG. 4) poured over the previously-formed refractory material 34 up to the level 40. This, of course, forms the lower base portion 16 of FIG. 1.

While a non-refractory stone material may be used for the base portion 16 as described above, it is preferable to also form the base from the same refractory material as the tooth die itself. In this latter case, the die is not removed from its cavity 30 until the refractory material has been poured over it to form the base 16. In either case, firing of the refractory material takes place at about l900- F. to 2000 F. for maturation of the materials system and to yield a model having dimensions within +0.30.0% of the original tooth preparation.

As was mentioned above, the dowels 36 previously used comprise solid brass rods or pins. These pins and, for that matter, any metallic pin, have a much higher coefficient of thermal expansion than the refractory material 34. Consequently, solid rod-type dowels cannot be used for this application since their bulk expansion on heating will produce stresses at the ceramic-dowel interface and fracture the ceramic die which has a much lower coefficient of thermal expansion.

In accordance with the present invention, the problem of differential thermal expansion is solved by providing a dowel of tubular construction on the principle that, while the coefficient of thermal expansion may be the same as a solid rod, the bulk expansivity will be less.

Several possible embodiments of the tubular dowel configuration of the invention are shown in FIGS. 6A6E. Thus, in FIG. 6A, the dowel simply comprises a tube 42 having cutout portions 44 and 46 at its upper portion where it is inserted into the refractory material such that the refractory material, in slurry form, will flow into the thus-formed openings and thus aid as an anchor in retaining the dowel in the tooth die. Note that there are no openings in the lower portion of the dowel such that it can be readily removed from the base 16, but not the tooth die itself. In FIG. 6B, the dowel is in the form of a hollow cone 48 also having a cutout portion 50 at its upper end which is inserted into the ceramic material such that the greater rate of expansion of the metal from which the dowel is formed is compensated for. In FIG. 6C, the dowel 52 is again tubular in construction, but in this case, the walls are crimped as at 54, again to permit the dowel to deform inwardly on heating and thus to compensate for differential thermal expansion. In FIG. 6D, the dowel comprises a split tube or cylinder 57 which is overlapped at one point about its periphery as at 58. Again, cutout portions 60 may be provided to assist in retaining the dowel in the tooth die. Finally, in FIG. 6E, the dowel 62 is again hollow tubular in construction with a conical lower end and a flared upper end 64 which again assists in retaining the dowel in the tooth die but not the base 16. Instead of providing openings or a flared upper end, it is also possible to employ pins or other projections extending outwardly from the dowel. This, however, complicates the fabrication of the dowel and, thus, the constructions of FIGS. 6A6E are preferable.

Although the invention has been shown in connection with certain specific embodiments, it will be readily ap parent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. In combination, a mass of castable refractory material and a dowel inserted into said material, the dowel being inserted into the material before it has set to form a hard mass and comprising a generally tubular metallic member having a degree of internal flexibility which will prevent fracture of the cast ceramic material upon heating due to a greater rate of thermal expansion of the metallic dowel than the cast ceramic, the mass of refractory material comprising a mixture including magnesium oxide and ammonium dihydrogen phosphate, the mixture being further mixed with a colloidal silica solution to give a thixotropic slurry which is poured into a mold cavity.

2. The combination of claim 1 wherein said hard mass is in the form of a model of a tooth.

3. The combination of claim 1 wherein the side walls of the tubular member are crimped.

4. The combination of claim 1 wherein the tubular member is provided with cutout portions to enhance re tention and reinforcement of the dowel in the die mass.

5. The combination of claim 1 wherein the tubular member is split along its axis and has overlapping edge portions.

6. The combination of claim 1 wherein the tubular member is conical in configuration.

References Cited UNITED STATES PATENTS 1,639,782 8/1927 Nies 32-2 1,867,300 7/1932 Bailey 3240 3,153,283 10/1964 Weissman 32-40 ROBERT PESHOCK, Primary Examiner. 

